It is often useful to add a user-specific data channel to a media stream, such as for watermarking, in order to provide a method for identifying the source of a copyright breach. Such channels can also be used for steganographic purposes. However, providing a specific watermark for each user typically involves that the watermark be applied to the original media prior to or during encoding. As a result, adding new users involves specific per-user re-encoding. This is not feasible in practice.